You are looking at
1
-
10
of
30
items for

G. A. LINCOLN

MRC Unit of Reproductive Biology, 2 Forrest Road, Edinburgh EH1 2QW

(Received 2nd July 1974)

The antlers of red deer, Cervus elaphus, are cast in the spring and a new set develop during the summer months. This cycle of antler renewal is controlled by seasonal changes in testosterone secretion by the testes (Lincoln, Youngson & Short, 1970; Lincoln, 1971). Low levels of circulating testosterone are associated with the casting and regrowth of the antlers, while the high androgen levels in late summer and autumn lead to the maturation of the new antler and the associated shedding of velvet.

During the period of antler growth, testosterone is present in the peripheral blood of stags in very low concentrations ( >200 pg/ml), but the hormone is physiologically active at this time and is associated with the normal development of the fully formed antler. If the testosterone levels are artificially modified by castration or

G. A. Lincoln

Summary. Blood samples were collected from adult Soay rams at frequent intervals during the regressed, developing and active phases of the sexual cycle, or after i.v. injections of synthetic LH-RH or standard preparations of ovine FSH and LH, and were assayed for FSH, LH and testosterone.

The highest FSH and LH levels occurred during the developing stage of the cycle, and the highest testosterone levels during the active stage. At each phase there were conspicuous short-term fluctuations in the concentrations of all three hormones, and episodic peaks of plasma LH were associated with increases in both FSH and testosterone. Synthetic LH-RH (5 μg) induced a rise in the plasma values of FSH and LH at all stages and the magnitude and duration of the response could be predicted from a knowledge of the endogenous short-term fluctuations. When 50–250 ng LH-RH were given i.v. plasma LH increased in a way similar to that occurring spontaneously. The ratio of FSH: LH released after LH-RH changed with the stage of the sexual cycle, but the clearance rate of the two hormones was not affected.

These findings are consistent with the control of FSH and LH by a single releasing hormone which is secreted in pulses. The different temporal patterns in the circulating FSH and LH result from differences in secretion and clearance.

G. A. Lincoln

In the mature ram pulses of luteinizing hormone (LH) are released from the anterior pituitary every few hours randomly throughout the day to appear in the peripheral circulation as transitory surges in the hormone concentration (Katongole, Naftolin & Short, 1974; Sanford, Winter, Palmer & Howland, 1974). Since the episodic secretory patterns recorded in the peripheral blood develop during puberty (Foster, 1974) and change in relation to the breeding season (Lincoln, 1976), they clearly have some bearing on the functional state of the testes. The purpose of the present study was to investigate the episodic nature of LH secretion in sheep exposed to different photoperiods.

Rams of the Soay breed were selected since they have conspicuous seasonal changes in reproductive physiology (Lincoln, 1976) and behaviour (Jewell, Milner & Morton Boyd, 1974). Six mature rams, 3-5 years of age and

G. A. Lincoln

Summary. Seasonal cycles were monitored in groups of wild (mouflon), feral (Soay) and domesticated breeds of sheep (Shetland, Blackface, Herdwick, Norfolk, Wiltshire, Portland, Merino, Soay × Portland and Soay × Merino) living outdoors near Edinburgh (56°N). Changes in the blood plasma concentrations of prolactin and FSH, and growth of the horns and pelage were measured every half calendar month from 1 to 3 years of age. In all breeds there was a clearly defined seasonal cycle in the plasma concentration of prolactin with an 18–66-fold increase in mean values from the nadir in November and December to the peak in May and June. The seasonal increase in prolactin was closely correlated with the seasonal increase in the growth of the horns, both within and between breeds (e.g. time of peak prolactin vs horn growth for 11 breeds, R = 0·62, P < 0·05). In the mouflon, Soay and some of the domesticated breeds of sheep (Wiltshire, Herdwick and Shetland), the seasonal increase in prolactin was also temporally correlated with the resurgence of growth of the pelage in spring and a conspicuous moult. In the other breeds developed to produce fine wool (e.g. Norfolk, Portland and Merino), there was no clear seasonal change in the pelage and growth continued throughout the year. Comparison between breeds indicated that continuous growth of the pelage was associated with higher plasma prolactin concentrations in winter. The times of the seasonal changes in plasma concentrations of prolactin were not significantly correlated with the corresponding changes in the plasma concentrations of FSH.

The overall results are consistent with a role for prolactin related to the growth of the horns and pelage rather than the seasonal cycle in reproduction. The differences between the wild-type and the domesticated breeds in the pelage represent the effect of selective breeding to produce a long fine fleece which has involved changes in both the seasonal pattern of prolactin secretion and the growth characteristics of the hair fibres.

G. A. Lincoln

Summary. Eight adult rams were housed in 16L:8D for 16 weeks and then exposed to short days (8L:16D) or 'skeleton' short days (11L:1D:5L:7D) for 16 weeks before being returned to long days. The 'skeleton' treatment promoted testicular development and regression in a way similar to that occurring in 8L:16D, indicating that a change in the total quantity of light is not a prerequisite for the photoperiodic response in the ram.

G. A. Lincoln

Summary.

During the autumn months, the gonads and reproductive tract of adult male hares (Lepus europaeus) are regressed and circulating gonadotrophin levels are low. At this time the pineal glands are most active as judged by the organ size, and the nuclear and cytoplasmic size of the pinealocytes. There was an inverse relationship between the size of the pineal gland and the weight of the testis, the plasma and testicular testosterone levels, and possibly also the plasma LH levels.

G. A. Lincoln

Summary. Red deer stags were infused continuously with the LHRH agonist buserelin at 180–270μg/day (1·2–1·8 μg/kg/day) for 72 days starting in late winter with the aim of suppressing reproductive function and inducing premature casting of the antlers. Contrary to expectation, the treatment resulted in a long-term stimulation of testicular activity lasting at least 2 months; the increases in plasma concentrations of testosterone were associated with an increase in aggressive behaviour and the development of rutting odour in the urine. The stags cast their antlers at the normal time in spring after the end of the treatments. The results indicate that the pituitary gonadotroph cells in the stag can continue to secrete LH in response to chronic exposure to an LHRH agonist and do not become rapidly desensitized. The effect of the agonist is therefore to cause significant stimulation of testicular activity which is a conspicuous response in the non-breeding season when the stags are already in a hypogonadal state.

G. A. LINCOLN

Summary.

The changes associated with puberty were studied in thirty-one red deer stags between 3 months and 3 years old. The majority of deer were born in June.

During the first 11 months of life, there was a gradual proliferation of spermatogonia in the seminiferous tubules. From May to October, between 11 and 16 months of age, a marked increase in the size and weight of the testes followed, the content of testosterone also began to rise and spermatogenesis was initiated. Over the same period, the weight of the epididymides, seminal vesicles, ampullae and prostate, and the concentration of seminal fructose increased. The antler pedicles began to develop in May, and antler tissue was evident by August.

The testes and accessory glands reached a peak in activity in October at 16 months of age but the development of the secondary sexual characteristics, including growth of the neck mane and 'stag' winter coat and the cleaning of the antlers, was not completed until 2 to 3 months later. Unlike the adult stags in October, the yearlings showed no rutting behaviour.

Following this peak in development, the testosterone content of the testes quickly declined to a low level, though spermatogenesis continued at a reduced rate through the winter and spring. Over this period, the weights of the accessory glands declined. In June, at 2 years old, spermatogenesis was arrested, the accessory glands were involuted, and the first set of antlers was cast. This marked the end of the first reproductive cycle; the testes and accessory glands redeveloped in October. In this second cycle, the secondary sexual characteristics developed earlier and some aspects of rutting behaviour occurred.

Puberty in the stag was defined as the period from 9 to 15 months of age, from the onset of androgen secretion to the completion of spermatogenesis. These changes were considered similar to the changes which occur each year in the adult in preparation for the breeding season.

D. Mortimer and G. A. Lincoln

Summary. Testes from 4 sexually active and 4 sexually regressed adult Soay rams housed under artificial lighting were examined. The reduction in testis weight during sexual regression (reduced to 35% of active) was due to a decrease in the diameter of the seminiferous tubules with extensive involution of the basement membrane, a marked reduction in the number of germ cells maturing beyond the spermatocyte stages, and a general disorganization of all but the most basal region of the seminiferous epithelium. The Sertoli cells contained large lipid droplets and deposits of lipochrome pigment and large necrotic bodies. The Leydig cells showed some reduction in content of smooth endoplasmic reticulum during sexual regression although they were approximately 30 times less active steroidogenically as judged by the peripheral levels of testosterone. Blood levels of LH and FSH were significantly reduced compared to those in sexually active animals, while the prolactin levels were much increased.

G. A. Lincoln and W. Davidson

Summary. Six adult Soay rams were housed under artificial lighting conditions, with alternating 16-week periods of long (16 h light: 8 h darkness) and short days (8L: 16D). During long days the rams were reproductively quiescent: the abrupt change from long to short days induced a specific succession of responses in the reproductive system. Plasma LH and FSH levels began to increase after 2–4 weeks, followed almost immediately by a rise in plasma testosterone levels accompanied by growth of the testes. Testicular activity continued to increase during short days and the greatly elevated androgen levels apparent after 5–10 weeks caused changes in the peripheral target organs, including growth of the epididymides, development of the sexual flush on the exposed ventral skin and heightened genital sensitivity. High testosterone levels were also associated with an increase in aggressive (scored by a mechanical device) and sexual (incidence of Flehmen) behaviour which was at peak about 1 month after the start of the peak androgen levels. The change to long days was associated with a decrease in plasma gonadotrophin levels within 2 weeks followed by a progressive decline in all reproductive parameters measured. Implantation of a low dose of testosterone (200 mg) during the period of reproductive quiescence induced the development of the sexual flush and an increase in genital tactile sensitivity, although behaviour was not significantly affected.

The annual changes in reproductive physiology and behaviour of 12 Soay rams living under natural lighting conditions were recorded for comparison with the experimental situation. The nadir of the sexual cycle was in the spring and early summer, and the sequence of events culminating in the mating season in the autumn was similar to that induced experimentally.